This invention relates to an exposure apparatus and, more particularly, to a projection exposure apparatus for projecting a pattern formed on an object such as a photomask or a reticle (which hereinafter will be referred to simply as "mask") onto another object such as a semiconductor wafer having a photoresist layer and for aligning the mask with the wafer at a high accuracy.
Recent demands for higher capacities of circuits in semiconductor devices have forced dramatic development of step-and-repeat type exposure apparatuses having high-resolution projection lenses for printing integrated circuit patterns onto semiconductor wafers. The projection lens employed in the step-and-repeat type exposure apparatus usually has a relatively small view field of the order of 10.times.10-15.times.15 mm.sup.2. For this reason, to transfer the integrated circuit pattern onto the entire surface of the wafer having a 5 or 6 inch diameter requires a large number of repetitions of alignment between the mask and the wafer and exposure of the wafer. Therefore, in order to improve the throughput of the apparatus employing the step-and-repeat system, how to achieve high-speed and high-accuracy alignment is one of the critical problems. The high-accuracy alignment can be most stably attained in a case where the system is arranged to view an alignment mark or pattern through the projection lens for pattern printing. In such case, it is desirable to use different wavelengths for the exposure and for the alignment. For example, a deep ultraviolet beam supplied by a super Hg lamp is used for the exposure while a laser beam providing high-luminance illumination is used for the automatic alignment. Alternatively, in order to prevent sensitization of the resist material by the alignment beam, a wavelength which is different from that for the exposure and to which the resist material is insensitive is used for the alignment.
Where different wavelengths are used for the exposure and for the alignment, it may be necessary to employ a projection lens system in which aberrations are corrected with respect to both of the exposure wavelength .lambda.1 and the alignment wavelength .lambda.2. In such case, the dependence of the focus position on the wavelength has a characteristic such as shown in FIG. 3 wherein the abscissa designates the focus position while the ordinate designates the wavelength. It is seen in FIG. 3 that the portion of the curve across the wavelength .lambda.1 shows a steep inclination. This means that, while the wavelengths .lambda.1 and .lambda.2 define the same correct focus position, a wavelength which is deviated from the wavelength .lambda.1, even if the deviation is only through a minute amount, would cause a large displacement of the focus position. If the expansion of the spectrum of the light supplied from the light source for the exposure is large, the imaging performance of the projection lens is remarkably deteriorated. As a solution for preventing this, the aberration-correction for the projection lens may be effected so as to provide a focus position curve as denoted by A in FIG. 4 to thereby prevent a large focus displacement due to the wavelength deviation. In such case, however, the focus position with respect to the alignment wavelength .lambda.2 disadvantageously displaces by a great amount. In order to prevent this, the wafer may be moved vertically upon alignment so that the alignment beam is focused accurately on the wafer surface. However, this tends to cause horizontal displacement of the wafer which makes it quite difficult to maintain the alignment accuracies.
The same assignee of the subject application has already proposed in U.S. Pat. No. 3,897,138 an interchangeable lens arrangement wherein a part of the element lenses of the projection lens system is replaced by an additional element lens to compensate for the focus displacement due to the difference in wavelength between the exposure beam and the alignment beam. While this arrangement shows satisfactory results, there still remains anxiety about deterioration, in stability, with age because to interchange the element lenses while preventing deviation from its place, inclination and misalignment with the optical axis and to frequently repeat the interchange such as in a case of a step-and-repeat type exposure apparatus would impose a substantial burden on the mechanism. On the other hand, there has been proposed to execute the alignment without the projection lens and at a station other than the exposure station. From the viewpoint of accuracies, however, it is preferable to effect alignment through the projection lens. In view of this, a proposal has been made in IBM Technical Disclosure Bulletin, Vol. 18, No. 2, July 1975, pages 385 and 386, to use the same wavelength for the alignment beam and the exposure beam and to employ an arrangement to prevent the area on the wafer, onto which the actual device pattern is to be transferred, from being exposed by the alignment beam. Use of the same wavelength however involves inconveniences. That is, in a case where the photoresist layer shows a high absorption factor with respect to the exposure wavelength, the amount of alignment beam coming back from the wafer decreases which makes it difficult to achieve alignment.